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Q: FAQ: How do I tune the PCB antenna when I have based my design on one of your reference designs? by 613920The antenna should be extended beyond the length used in the reference design to allow for antenna tuning. The antenna length in the reference design is the length we found to be optimal after our own tuning process. We consider it to be the optimal length for the reference circuit. The optimum length for a customer design could be different due to the different environment (PCB and especially changed ground reference). The resulting length can be longer or shorter than our antenna. Note that if the final equipment is encapsulated in plastics or used close to the human body this environment should also be used during the tuning process.
The tuning process is based on measuring the performance for different antenna lengths. The reduction in antenna length is physically done by cutting it down by a scalpel. This process should reveal the optimum antenna length for the current design, indicated as an optimum in performance. The approach of gradually reducing the antenna length explains why the antenna length should be extended beyond the expected length during first prototype PCB.
The measured performance can be output power or received signal strength. The performance of the antenna matching circuit and antenna efficiency is then measured together. This approach often fails to identify the overall optimum performance. The advantage is that it requires less equipment.
For optimum performance we suggest to measure the antenna impedance and tune the real part of the impedance as close to 50 ohms as possible. The imaginary part of the impedance can then be removed using a matching network. Other antennas might have different optimum impedances. In that case the matching circuit has to transfer the impedance real part as well.
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